1. Field of the Invention
The present invention relates to an optical transmission system which amplifies a main signal light using Raman amplification, and in particular an optical transmission system comprising a function for measuring the power of noise light generated by Raman amplification and making corrections.
2. Description of the Related Art
As a conventional optical transmission system, for example, there is known a system where wavelength division multiplexed (WDM) signal light which is made by multiplexing a plurality of beams of signal light with different wavelengths, is sent from a sending terminal station to an optical transmission line. Then the WDM signal light is repeatedly transmitted to a receiving terminal station while being amplified by a plurality of repeater stations (optical amplifiers) arranged on the optical transmission line. Regarding such a system, for example there is proposed a technique which measures the input light power to the optical amplifiers, calculates the power of noise light generated in the local station on the basis of the results of the measurement, and then corrects with respect to an output setting level of an automatic level control (ALC), to keep the signal light power per wavelength included in the output light of the optical amplifiers at a constant regardless of the number of wavelengths. This is done to improve the transmission characteristic (for example, refer to Japanese Unexamined Patent Publication No. 2000-232433).
However, the abovementioned conventional technique has the following problems. That is, in the conventional technique, when correcting with respect to the output setting level, the noise light power generated in the local station is obtained by calculation based on: a measured value by a moniter provided in an input section of the optical amplifier, specifically a measured value of the input light power corresponding to the sum of the WDM signal light power which is inputted into the optical amplifier, and the noise light power generated in the repeater station of the preceding stage; various characteristics of the optical amplifier corresponding thereto; and the number of wavelengths of the WDM signal light. Since as mentioned above, the measured value of the noise light power is the sum of the signal light power and the noise light power, the calculated value of this noise light power can only be obtained by estimation taking into account the corrected value of the output setting level in the repeater station of the preceding stage. Accordingly, there is a problem in that the corrected value of the output setting level of the local station obtained by using the calculated value of the noise light power as mentioned above, includes an error, and the error is accumulated as the number of the repeater stations is increased.
Moreover, if the abovementioned conventional technique is applied to a system which has been drawing attention recently, which jointly uses Raman amplifiers to repeatedly transmit the WDM signal light, it is necessary to correct the noise light due to the stimulated Raman scattering which is caused by the pumping light supplied to the Raman amplification medium arranged between the repeater station of the preceding stage and the local station. In the following description, the noise light due to the stimulated Raman scattering is called ASS (Amplified Stimulated Raman Scattering) light. Regarding the correction of such ASS light as well as the case of correction of the noise light generated in the repeater station of the preceding stage mentioned above, it is difficult to measure the generation state separately from the WDM signal light. Therefore, the power of the ASS light is estimated by calculation using predetermined coefficients and the like. Accordingly, there is a problem in that arithmetic processing is complicated and becomes an error factor at the same time.
Incidentally, as a well-known technique for measuring the noise characteristics of an optical amplifier, there is proposed a technique for example, where the signal light inputted into the optical amplifier is given a required pulse modulation. A light cutoff device is provided on the output side of the optical amplifier, so that the output light is received while operating the light cutoff device in synchronism with the input light level so as to measure the noise characteristics. Where an optical coupler is provided on the output side of the optical amplifier to branch the output light into two, then one part of the output light is delayed until they do not correlate with each other to make it a local oscillating light from the station, and heterodyne detection is performed using a dual balance type optical receiver, so as to measure the noise characteristics (for example, refer to Japanese Unexamined Patent Publication No. 5-257177).
If the noise light is corrected as mentioned above by applying this well-known technique, it becomes possible to actually measure the power of the noise light locally generated in the local station by arranging the light cutoff device and the like on the output side of the optical amplifier. However, it is not possible to measure the power of the noise light generated on the upstream side from the local station, specifically the ASS light which is generated in a distributed manner on the optical transmission line between the repeater station of the preceding stage and the local station. Therefore, it is difficult to realize correction of the noise light with high accuracy. Moreover, since the noise light is measured by giving the input light to the optical amplifier a required pulse modulation to synchronize it with the light cutoff device, or by using the dual balance type optical receiver, there is also a disadvantage in that a complicated configuration and complicated processing are required.